A molten parison on a core rod of an injection blow molding machine can be blown into contact with the wall of the blowing mold cavity more quickly if the air pressure outside of the parison is a partial vacuum. This is particularly true in large mold cavities where there is a substantial amount of air which must be displaced from the cavity by the expanding parison. A partial vacuum in the blowing mold between the article being blown and the wall of the cavity helps prevent the plastic from pulling away from contact with the wall of the cavity as the plastic cools and shrinks. By maintaining closer contact of the plastic with the wall, the blown article cools faster and can be ejected from the blowing mold after a shorter cooling period.
Another feature of this invention is the making of blown ware with foamed or expanded plastic. By putting foaming agent in the plastic, a myriad of closed gas cells are formed in the plastic, and these cells are extremely small as long as the plastic is under the high injection pressure of an injection molding machine, but the gas cells expand in size as the pressure decreases, and this increases the thickness of the plastic that forms the parison on the core rod.
When the plastic material is in the blowing mold, this invention controls the size of the bubbles by changing the difference in pressure on the inside and outside of the blown article. The size to which the gas cells will expand depends on the pressure to which they are subjected while the plastic is in a molten condition. This pressure can be reduced by drawing a partial vacuum in the blowing mold around the outside of the plastic parison as it expands during the blowing operation.
The final size of the gas cells depends on the pressure differential across the wall of the blown article at the time that the plastic solidifies and the size of the gas cells becomes set.
The preferred embodiment of the invention involves injection molding; but the invention can also be used for extrusion molding.
Other objects, features and advantages of the invention will appear or be pointed out as the description proceeds.